Device for the production of visible or photographic images with the aid of a beam of neutrons as depicting radiation



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HARTMUTISRAEL.KALLMANN,

THE AID 0F A BEAM OF NEUTRONS AS DEPICTING RADIATION Filed Sept. 28, 1940 m u n Sept. 29, '1942.

FORMERLY KNOWN AS HARTMUT KALLMAN, DEVICE FOR THE'PRODUCTION OF VISIBLE OR PHOTOGRAPHIC IMAGES WITH Patented Sept. 29,1942

s'V PATENT OFFICE DEVICE FOR. THE PRODUCTION OF VISIBLE OR PHOTOGRAPIIIC IlVIAGES WITH THE AID OF A BEAM F ING RADIATION Hartmut Israel Kallmann,

Kallmann,

Hartmut NEUTRONS AS DEPICT- formerly known as Berlin-Charlottenburg, Y and Ernst Kuhn, Berlin, Germany;

the Alien'Prope'rty CustodianY vested in Application September 28, 1940. Serial No. 358,939 In Germany September 29, 1939 invention relates to the art of producing sible-'or photographic images with the aid of beam of neutrons as depicting radiation.

' Itfhas been proposed to produce visibleor photographicl images with employment of neutrons as depicting radiation therebythat, in a neutron-reactive layer in a converter of neutron images by the depicting neutrons heavy charged particles or electrons are produced and these liberate in the neutron-reactive layer or in an adjacent layer slow electrons which are accelerated by electric fields and produce on a fluorescent screen, or on a photographic layer, an image after they have passed through an electron-optical depicting system. This proposed arrangement possesses under certain circumstances the inconvenience that it reacts also on neutrons of great velocity in a slight measure. This may become disturbing especially if, in consequence of the screening out of a narrow beam of slow neutrons, the amount of the quick neutrons having partly penetrated through the absorption layers of the screening device and impinging on to the image converter is comparatively great. The quick neutrons exert also, though in a low measure, an effect upon the neutron-reactive layer, in that they liberate from this layer recoil nuclei which in turn generate slow electrons: these slow electrons are also accelerated by the electric fields and cause fogging of the iiuorescent screen image or of the photographic image.

By recoil nuclei are meant nuclei which have been impinged on by quick neutrons and thereby have acquired a considerable kinetic force.

The inconveniences above noted may be avoided in the arrangement according to the invention thereby that between the neutron-reactive layer and the layer from which the slow electrons are liberated an absorption layer is interposed, which absorption layer if possible contains only heavy nuclei, for instance, of lead. The thickness of the absorption layer may be selected so that it is suiciently permeable for the heavy charged particles or electrons which have been produced in the neutron-reactive layer by the slow neutrons, whereas they are particularly impermeable to the recoil nuclei which can be stopped much more easily. Consequently, only the eiiects are intensified in the image converter which are caused by the slow neutrons. The recoil nuclei produced in the absorption layer which contains nuclei as heavy as possible are practically ineffective, as owing to their large mass they possess only an extraordinarily small 3 Claims. (Cl. Z50-83.3)

range and therefore proceed from the absorption layer only in a very little number.

The layer emitting slow electrons arranged on the separating layer is preferably selected to be as thin as possible, in order that in the same only very few recoil nuclei are produced. It may, for instance, be dusted upon the separating layer. The method according to the invention is especially important in those cases in which the neu- ,electrons are produced. The intensity of the heavy charged particles or electrons thus produced is different from place to place in the cross-section of the beam as the neutron radiation is locally weakened by the body 3 to be depicted. The heavy charged particles or electrons liberate slow secondary electrons from the layer 6, which layer may consist, for instance, of cesium and have a thickness of about 0.0001 mm. The rapid neutrons liberate recoil nuclei from the neutron-reactive layer 5.

According to the invention, these recoil nuclei are stopped by the absorption layer 23, which contains as far as possible only heavy nuclei and consists, for instance, of lead, so that the recoil nuclei cannot advance to the layer 6. The thickness of the layer 23, which advantageously may consist of lead and have a thickness of about 0.0003 mm. is chosen so that the particles which are liberated in the neutron-reactive layer 5 by the slow neutrons attain the layer 6 from which they liberate slow secondary electrons which latter are emitted into the vacuum space 'l and there are accelerated by an electric iield existing for instance between the electrode 8 and the layers 5 or 6 respectively. The so-accelerated secondary electrons produce an image on a iluorescent screen, or on a photographic layer, I0 after they have passed through an electronoptical image-producing system, for instance through a magnetic or electrostatic lens 9.

The image produced on the fluorescent screen, or on the lphotographic layer, I0 represents an image. rich in contracts and intensity, of the object 3, which circumstance is due to the absorption conditions for neutrons in this body. Without the absorption layer 23 provided according to the invention the recoil nuclei proceeding from the neutron-reactiVe-layer -5 would reach the layer 6 and liberate from this layer slow secondary electrons: these secondary electrons would be accelerated and photographed Just as are the secondary electrons which are liberated from the layer 6 by the particles liberated from the neutron-reactive layer 5 by the slow neutrons. The image of the4 intensity distribution of the disturbing quick neutrons would thus overlie the desired image of the intensity distribution of the slow neutrons. This logging of the desired image is avoidedby the stopping of the recoil nuclei in the absorption layer 23 provided according to the invention, the liberating of these recoil nuclei being eifected merely by the quick neutrons.

In the drawing Fig. 1 shows also a further arrangement for increasing the intensity and for improving the contrasts for the case in which the layer I represents a fluorescent screen which may consist, for instance, of zinc sulphide and have a thickness of about 0.005 mm. 'I'he radiation emitted by this uorescent screen acts upon the adjacent light-sensitive layer II, which may consist of caesium, about 0.0001 mm. thick, and liberates from this layer photo-electrons, which in known manner are accelerated by the electric iield, existing between layer II and electrode I2, and copied on the fluorescent screen I4 by means of an electron-optical copying system, for instance a magnetic lens I3. The image produced on the iiuorescent screen I4 can be strengthened by a second image intensifier of this kind. 'I'he vacuum spaces 1 and I5 may communicate the one with the other.

In the iiuorescent screens I0 and I4 it is advisable to provide reecting layers I6 and I1, which may consist of aluminum and have a thickness of about 0.0002 mm., on the sides remote from the observer, these layers being permeable :for the impinging electrons and reflecting the light emitted from the uorescent screens to the side at which the observer stands. In case the arrangement be such that the heavy charged particles or electrons liberated from the neutron-reactive layer by means of the neutrons act upon an adjacent fluorescent mass I8 (which may for instance consist of zinc sulphide and have a thickness of about 0.005 mm), the radiation of which liberates slow electrons from an adjacent light-sensitive layer I9 (e. g., of cesium, about 0.0001 mm. thick), as shown in Fig. 2, it is advisable to provide also a reiiecting layer 20 (e. g., aluminum, about 0.0002 mm. thick) betweenthe absorption layer 24 (similar in composition and thickness to layer 23 above) and the fluorescent substance I8.

Under some circumstances it is advisable to separate the photocathode II from the iluorescent screen I0 by a light-permeable part 2I of the vessel wall. In this case the two layers I0 and II need not lie on the same potential. It may cording to the invention are much sharper than those made with the known arrangements, for the reason that the slow electrons used for the images are taken only from an extraordinarily thin surface layer of the neutron-reactive layer or of the layer emitting the slow electrons.

We claim:

1. A device for the production oi visible or photographic images with the aid of a beam oi slow neutrons as depicting radiation which contains also quick neutrons and of a neutron image converter as depicting system for the intensity distribution of incoming neutron radiation comprising in combination with a neutron reactive layer emitting charged particles under the influence of slow neutrons, a layer emitting, slow secondary electrons under the action of the charged particles, and an absorption layer rich in heavy nuclei interposed between said layers.

2. A device for the production of visible or photographic images with the aid of a beam of slow neutrons as depicting radiation which contains also quick neutrons and of a neutron image converter as depicting system for the intensity distribution of incoming neutron radiation comprising in combination with a neutron reactive layer emitting charged particles under the influence of slow neutrons, a layer emitting slow secondary electrons under the action of the charged particles and an absorption layer rich in heavy nuclei interposed between said layers, said absorption layer being of such thickness that it lets pass without loss of energy the particles produced by the slow neutrons in the neutron-reactive layer while stopping, however, recoil nuclei which may be liberated from the neutron-reactive layer by the quick neu `ons, so that such recoil nuclei cannot liberate electrons from the layer capable of emitting secondary electrons.

3. A device for the production of visible or photographic images with the aid of a beam of slow neutrons as depicting radiation which contains also quick neutrons and of a neutron image converter as depicting system for the intensity distribution of incoming neutron radiation comprising in combination with a neutron reactive layer emitting charged particles under the inuence of slow neutrons, a layer emitting slow secondary electrons under the action of the charged particles and an absorption layer rich in heavy nuclei interposed between said layers, said layer capable of emitting secondary electrons being arranged on the absorption layer and having so little thickness that only very few recoil nuclei are formed in the same.

HARTMUT ISRAEL KALLMANN, Formerly knoum as Hartmut Kallmann.

ERNST KUHN. 

